1. Field of the Invention
This invention relates to a stabilizer for an automotive vehicle, and more particularly to a controllable stabilizer.
2. Description of the Prior Art
Automotive vehicles of the independent suspension type are generally equipped with stabilizers to reduce inclination or roll of the vehicle bodies during such conditions as turns of the vehicles. The stabilizer is usually connected between the suspension arms of the left-hand and right-hand vehicle wheels. When the left-hand and right-hand wheels are in similar positions with respect to the suspensions, the stabilizer does not twist so that the suspensions are mutually independent. When one of the left-hand wheel and the right-hand wheel passes over a projection on the road surface, or when the vehicle turns and thus the left-hand wheel and the right-hand wheel assume considerably different positions with respect to the suspensions, a torsion bar in the stabilizer is twisted. The reaction of the twisted torsion bar induces a torsional resilient force in the direction of cancelling the twist of the torsion bar so that the positions of the two wheels with respect to the suspensions are equalized. The function of the stabilizer affects the rolling characteristic of the vehicle and the road-surface-following characteristic of the vehicle wheels.
In a typical stabilizer, the rolling characteristic of the vehicle is incompatible with the road-surface-following characteristic of the vehicle wheels. Accordingly, a compromise is performed in setting these two characteristics. In other words, these two characteristics are set to compromised fixed levels respectively.
It is desirable that the torsional resilient force in the stabilizer can be adjusted in accordance with running or travelling conditions of the vehicle. Specifically, it is desirable to reduce the torsional rigidity during straight travel of the vehicle and to increase the torsional rigidity during turn of the vehicle. It should be noted that the reduced torsional rigidity enhances the road-surface-following characteristic of the vehicle wheels while the increased torsional rigidity enhances the rolling characteristic of the vehicle.
Japanese published unexamined patent application No. 57-66009 discloses an automotive stabilizer control system including a hydraulic cylinder provided to the connection of a stabilizer to a suspension arm. The hydraulic cylinder has a movable piston, and two pressure chambers at opposite ends of the piston. A bypass passage connects these two pressure chambers. A check valve selectively blocks and unblocks the bypass passage. When the check valve blocks the bypass passage, the hydraulic cylinder assumes a rigid state which induces a great torsional rigidity of the stabilizer. When the check valve unblocks the bypass passage, the hydraulic cylinder assumes an easily deformable state which induces a small torsional rigidity of the stabilizer. The check valve responds to a hydraulic pressure which drives a power steering apparatus. The power steering hydraulic pressure depends on whether the vehicle is running straight or turning. When the vehicle is running straight, the check valve unblocks the bypass passage so that a small torsional rigidity of the stabilizer is induced. When the vehicle is turning, the check valve blocks the bypass passage so that a great torsional rigidity of the stabilizer is induced. This prior-art stabilizer control system therefore provides only rudimental control.